1. Technical Field
This invention generally relates to object oriented programming and more specifically relates to an apparatus and method for updating objects in an object oriented system.
2. Background Art
The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely sophisticated devices, and computer systems may be found in many different settings. Computer systems typically include a combination of hardware, such as semiconductors, and circuit boards, and software, also known as computer programs. As advances in semiconductor processing and computer architecture push the performance of the computer hardware higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago.
Computer systems typically include operating system software that controls the basic function of the computer, and one or more software application programs that run under the control of the operating system to perform desired tasks. For example, a typical IBM Personal Computer may run the OS/2 operating system, and under the control of the OS/2 operating system, a user may execute an application program, such as a word processor. As the capabilities of computer systems have increased, the application software programs designed for high performance computer systems have become extremely powerful. Additionally, software development costs have continued to rise because more powerful and complex programs take more time, and hence more money, to produce.
One way in which the performance of application software programs has been improved while the associated development costs have been reduced is by using object oriented programming concepts. The goal of using object oriented programming is to create small, reusable sections of program code known as "objects" that can be quickly and easily combined and re-used to create new programs. This is similar to the idea of using the same set of building blocks again and again to create many different structures. The modular and re-usable aspects of objects will typically speed development of new programs, thereby reducing the costs associated with the development cycle. In addition, by creating and re-using a comprehensive set of well-tested objects, a more stable, uniform, and consistent approach to developing new computer programs can be achieved.
A central concept in object oriented programming is the "class." A class is a template that defines a type of object. A class outlines or describes the characteristics or makeup of objects that belong to that class. By defining a class, objects can be created that belong to the class without having to rewrite the entire definition for each new object as it is created. This feature of object oriented programming promotes the reusability of existing object definitions and promotes more efficient use of program code.
An object in an object oriented computer program typically has attributes defined by state data that determine how the object will behave. State data as used herein defines both methods and data with in an object, and is a concept that is well-known to one skilled in the art. If an object is transient, it is created within a process, and terminates when the process ends. If an object is persistent, however, mechanisms are put in place to allow the object to survive the process that creates it so it can be accessed by other processes.
Objects are typically made persistent by storing their state data in a local data store. In many known computer system, the process of making an object persistent is known as "externalization". Externalization is the means or protocol used in object oriented programming for transferring data out of an object. In essence the state data that defines the attributes of an object are "externalized", or written out of the object, into a different format that is easily stored in the local data store. When the object is needed again, the externalized state data is internalized into an object, creating an exact copy of the object as it previously existed.
Computer programs naturally evolve over time. Changing an object oriented program requires changes to objects. When changes to persistent objects are required, there is no uniform mechanism for updating the persistent objects. For example, assume a company has an Employee class that contains the employee's name; address, including 5-digit ZIP code; and home telephone number. Let's further assume that the employee class needs to be changed to incorporate a nine digit ZIP code and to include a department code for the employee. A new class is defined with the additional data fields for the ZIP code and the department. But how is the existing, persistent object updated so that it contains the additional data fields?
Referring to FIG. 2, one known method 200 for updating persistent objects first stores the state of all objects in the system in memory (step 210), generally by externalizing the object data to a data store. Next, all the objects in the system are passivated (step 220). The new class is loaded (step 230), an instance of the new class is created (step 240), and one or more methods on the new class are invoked to retrieve the stored state data (step 250), generally by internalizing the state data into the new instance(s) of the new class. While this method succeeds at updating the persistent objects, it requires that the system be shut down in order to passivate all the objects. With large systems that have thousands or millions of objects, this process can be very time-consuming, and can take a computer system off-line for many hours. Shutting down a large computer system for many hours is an unacceptable solution for many computer systems. Without a mechanism for easily updating persistent objects without shutting down, the computer industry will continue to suffer from inefficient and costly methods of updating persistent objects in an object oriented computer system.